We apply morality in specific situations.
In order to apply it correctly, we must be aware of all the relevant materials facts.
For instance, if morality requires us to engage only in the kind of sexual activity that is most likely to result in procreation, we have to know which kind of sexual activity is most likely to result in procreation.
Knowing all relevant truths is a necessary condition for doing the right thing, no matter which moral theory turns out to be true. So, is avoiding all relevant falsehoods
It follows that we have a duty to try to ensure that we believe all relevant true propositions, and to disbelieve relevant false ones.